OSI Model

Application
Presentation
Session
Transport
Network
Data Link
Physical

Diagram. The seven layers of the OSI model.

The OSI (Open System Interconnection) model consists of a seven-layer architecture that organizes the sending of data from hosts across a network. The OSI model was developed by the International Standards Organization (ISO) after the TCP/IP model to provide greater granularity of networking assignments within the model.

The layers are:

Physical Layer: This layer is responsible for the physical connections of the devices in the network. This layer is implemented through the use of devices such as hubs, repeaters, modem devices, and physical cabling.
Data Link Layer: This layer is responsible for the error-free delivery of data to the receiving device or node. This layer is implemented through the use of devices such as switches and bridge devices, as well as anything with a network interface, like wireless or wired network cards.
Network Layer: This layer is responsible for the transmission of data between hosts in different networks as well as routing of data packets. This layer is implemented through the use of devices such as routers and some switches.
Transport Layer: This layer provides services to the application layer and receives services from the network layer. It is responsible for the reliable delivery of data. It segments and reassembles data in the correct order for it to be sent to the receiving device. It may also handle the reliable delivery of data and any retries of data that are lost or corrupted (for example, TCP does this). This layer is often called the heart of OSI.
Session Layer: This layer is responsible for connection establishment, session maintenance, and authentication.
Presentation Layer: This layer is responsible for translating data from the application layer into the format required to transmit the data over the network as well as encrypting the data for security if encryption is used.
Application Layer: This layer is responsible for network applications (like HTTP or FTP) and their production of data to be transferred over the network.

The OSI model is widely used throughout networking documentation and discussions. Layers are often referred to by number, not name, so memorizing the numbers and having a good understanding of each layer’s uses are essential for success in the IT community.

OSI model example: Consider this example of the OSI model in action by viewing the layers in the table below.

	OSI Layers in Connecting to WGU's Website
Layer 7 \n	You open a web browser on your laptop and type www.wgu.edu into the search bar.
Layer 6 \n	The command is then sent to Layer 6 to be encrypted as it is sent (and decrypted when received by the webserver).
Layer 5	Establishes the connection between your laptop and WGU’s server and maintains it while you accomplish your tasks on the website.
Layer 4	Data is transferred and is then segmented and numbered to send the data across the network in sizes the network can handle (typically around 1,500 bytes maximum) and to reassemble it in the correct order by the webserver.
Layer 3	Receives the segments and transmits them across the network as packets. As you are accessing the website, Layer 3 adds the source and destination IP addresses to each individual data packet. The destination IP address will be the router on the network that will move data off the local network. The series of routers between that router and the destination server will determine how the packet is moved across various networks between them.
Layer 2	Receives the packets and adds physical addressing by adding sender and receiver MAC addresses to each data packet. This information forms a unit called a frame.
Layer 1	Receives the frames and data and sends them via the local media (copper wires, fiber-optic cables, etc.) to the switches, routers, etc., along the network path. All of this takes a matter of microseconds to achieve.